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Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="alignment"><a href="#alignment" class="header-anchor">#</a> Alignment</h1> <p>All types in C++ have an alignment. This is a restriction on the memory address that objects of that type can be created within. A memory address is valid for an object's creation if dividing that address by the object's alignment is a whole number.</p> <p>Type alignments are always a power of two (including 1).</p> <h2 id="controlling-alignment"><a href="#controlling-alignment" class="header-anchor">#</a> Controlling alignment</h2> <p>The <code>alignas</code> <a href="http://stackoverflow.com/documentation/c%2b%2b/4891/keywords" target="_blank" rel="noopener noreferrer">keyword<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> can be used to force a variable, class data member, declaration or definition of a class, or declaration or definition of an enum, to have a particular alignment, if supported. It comes in two forms:</p> <ul><li><code>alignas(x)</code>, where <code>x</code> is a constant expression, gives the entity the alignment <code>x</code>, if supported.</li> <li><code>alignas(T)</code>, where <code>T</code> is a type, gives the entity an alignment equal to the alignment requirement of <code>T</code>, that is, <code>alignof(T)</code>, if supported.</li></ul> <p>If multiple <code>alignas</code> specifiers are applied to the same entity, the strictest one applies.</p> <p>In this example, the buffer <code>buf</code> is guaranteed to be appropriately aligned to hold an <code>int</code> object, even though its element type is <code>unsigned char</code>, which may have a weaker alignment requirement.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">alignas</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span> <span class="token keyword">unsigned</span> <span class="token keyword">char</span> buf<span class="token punctuation">[</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token punctuation">(</span>buf<span class="token punctuation">)</span> <span class="token keyword">int</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p><code>alignas</code> cannot be used to give a type a smaller alignment than the type would have without this declaration:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">alignas</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">int</span> i<span class="token punctuation">;</span> <span class="token comment">//Il-formed, unless `int` on this platform is aligned to 1 byte.</span>
<span class="token keyword">alignas</span><span class="token punctuation">(</span><span class="token keyword">char</span><span class="token punctuation">)</span> <span class="token keyword">int</span> j<span class="token punctuation">;</span> <span class="token comment">//Il-formed, unless `int` has the same or smaller alignment than `char`.</span>
</code></pre></div><p><code>alignas</code>, when given an integer constant expression, must be given a valid alignment. Valid alignments are always powers of two, and must be greater than zero. Compilers are required to support all valid alignments up to the alignment of the type <code>std::max_align_t</code>. They <strong>may</strong> support larger alignments than this, but support for allocating memory for such objects is limited. The upper limit on alignments is implementation dependent.</p> <p>C++17 features direct support in <code>operator new</code> for allocating memory for over-aligned types.</p> <h2 id="querying-the-alignment-of-a-type"><a href="#querying-the-alignment-of-a-type" class="header-anchor">#</a> Querying the alignment of a type</h2> <p>The alignment requirement of a type can be queried using the <code>alignof</code> <a href="http://stackoverflow.com/documentation/c%2b%2b/4891/keywords" target="_blank" rel="noopener noreferrer">keyword<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> as a unary operator. The result is a constant expression of type <code>std::size_t</code>, <strong>i.e.,</strong> it can be evaluated at compile time.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"The alignment requirement of int is: "</span> <span class="token operator"><<</span> <span class="token keyword">alignof</span><span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span> <span class="token operator"><<</span> <span class="token string">'\n'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Possible output</p> <blockquote></blockquote> <p>The alignment requirement of int is: 4</p> <p>If applied to an array, it yields the alignment requirement of the element type. If applied to a reference type, it yields the alignment requirement of the referenced type. (References themselves have no alignment, since they are not objects.)</p> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>The standard guarantees the following:</p> <ul><li><p>The alignment requirement of a type is a divisor of its size. For example, a class with size 16 bytes could have an alignment of 1, 2, 4, 8, or 16, but not 32. (If a class's members only total 14 bytes in size, but the class needs to have an alignment requirement of 8, the compiler will insert 2 padding bytes to make the class's size equal to 16.)</p></li> <li><p>The signed and unsigned versions of an integer type have the same alignment requirement.</p></li> <li><p>A pointer to <code>void</code> has the same alignment requirement as a pointer to <code>char</code>.</p></li> <li><p>The cv-qualified and cv-unqualified versions of a type have the same alignment requirement.</p></li></ul> <p>Note that while alignment exists in C++03, it was not until C++11 that it became possible to query alignment (using <code>alignof</code>) and control alignment (using <code>alignas</code>).</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/alignment.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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